[wallet-ui] Login fail should happen before signature sent

[ldct]

No description provided.

[snario]

@ebryn this is an important security thing. We should check if there is a user first before signing a message and sending it to the server.

[joelalejandro]

@snario I think the Wallet UI has dragged the same unsafe implementation.

If I understand correctly, the protocol would be:

1. See if there is a user with the Metamask's ethAddress registered.
2. If the user exists on the database, request the signature. If not, return an error and not request the signature.
3. If the signature is valid, allow the user to login.